Overview
ISO 21896:2020 - Paper, pulp, and recycling - Decolouration test of dye coloured paper products and paper products printed using dye inks specifies a laboratory method to measure the decolouration capability of dye‑coloured papers and paper products printed with dye inks. The standard defines how to reslush (re‑pulp) samples, apply reductive and oxidative bleaching treatments, form filter pads and perform optical measurements to calculate a decolouration index (DRI). Note: this standard does not apply to papers printed with pigment inks.
Key topics and technical requirements
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Scope & principle
- Laboratory method to assess chemical decolouration (not mechanical deinking) of dye‑containing papers during recycling and bleaching.
- Produces a DRI value indicating bleaching effectiveness.
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Apparatus & conditions
- Pulp preparation mixer (e.g., 5 L Hobart‑type pulper).
- Reductive bleaching with sodium hydrosulphite (controlled pH, 50 ± 2 °C water bath, sealed jars).
- Oxidative bleaching with hydrogen peroxide (70 ± 2 °C water bath, pH control, sealed/plastic bag).
- Pad preparation: Büchner filtration, 150 mm filter papers (retaining ≥25 µm particles), Rapid‑Köthen drying or 95 ± 2 °C heating.
- Optical instrumentation to measure tristimulus/CIELAB values and diffuse reflectance.
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Procedure highlights
- Sampling per ISO 186 and pulp dry matter determination (ISO 638).
- Re‑slushing and controlled bleaching (both reductive and oxidative options).
- Preparation, conditioning and optical measurement of filter pads.
- Calculation of DRI (%) from measured optical changes.
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Safety & quality
- Use of hazardous reagents and temperature control - observe appropriate laboratory safety and handling.
Applications and users
ISO 21896:2020 is practical for:
- Paper and board recyclers assessing how dye‑coloured or dye‑printed feedstocks respond to bleaching.
- Quality control and R&D laboratories evaluating the effectiveness of bleaching chemistries on dyed papers.
- Ink and paper manufacturers characterizing dye fugacity and recyclability of products printed with dye inks.
- Environmental and process engineers optimizing recycling lines where bleaching supplements deinking.
Keywords: ISO 21896:2020, decolouration test, dye coloured paper, dye inks, paper recycling, decolouration index, sodium hydrosulphite, hydrogen peroxide, filter pads, optical measurements.
Related standards
Relevant normative references include:
Use ISO 21896:2020 to quantify bleaching potential of dye‑based paper products and to support recycling process decisions where chemical decolouration supplements deinking.